1. Field of the Invention
The present invention relates in general to control systems of an automatic transmission that has both a continuously variable transmission section and a stepwise variable transmission section, and more particularly to the control systems of a type that, upon need of a speed change of the automatic transmission, makes a control of the continuously variable transmission section cooperate with that of the stepwise variable transmission section thereby to achieve a quite smoothed speed change of the transmission as if the speed change is like a continuously variable speed change. More specifically, the present invention is concerned with the control systems of a type that, upon need of a speed change of the automatic transmission, makes the control of the continuously variable transmission section cooperate with a so-called “change-over shift change” of the stepwise variable transmission section. The change-over shift change means a speed change or shift change that is effected by disengaging one frictional element that has been kept engaged and engaging the other frictional element that has been kept disengaged. Such change-over shift change is sometimes called “replacement of gear shift”.
2. Description of the Related Art
One of the control systems of the above-mentioned type is disclosed in Japanese Laid-open Patent Application (Tokkaihei) 5-079554. In the control system of this publication, a technique is practically used for carrying out a cooperative control of the continuously variable transmission section during the change-over shift change of the stepwise variable transmission section, more specifically, in an inertia phase of the change-over shift change of the stepwise variable transmission section. That is, for suppressing or at least minimizing undesired shift shock that would be caused by a poor cooperative control of the continuously variable transmission section, starting of the inertia phase is detected or judged based on a change of input rotation speed of the stepwise variable transmission section.